Acidity in zeolites and their characterization by different spectroscopic methods

An overview of the different acidic sites in zeolites is presented here. It is followed by a discussion of the methods for identification of the various acidic sites and their distribution in some widely used zeolites such as faujasite, ZSM-5 and mordenite. Emphasis is laid on the discussion of various techniques, their relative advantages and disadvantages and several example studies, where these techniques are applied. The techniques discussed include titration in aprotic solvents, temperature-programmed desorption, microcalorimetric measurement of the heats of adsorption of probe molecules, IR spectroscopy with and without probe molecules, MAS NMR, photoelectron spectroscopy and position annihilation spectroscopy. From this study it appears that a single technique is not sufficient for the characterization and interpretation of zeolite acid sites and for the prediction of catalytic activity. Combination of different techniques such as NMR with LR can provide more information about the structure of acid sites of zeolites.